The present invention relates to molecules involved in cell-cell interactions in the immune system. In particular, the invention relates to a cell surface protein that contains certain classical cadherin characteristics, but it exhibits an apical distribution pattern on the surface of lymphocytes. The membrane location of this molecule correlates with the contact interface between T and B cells, and antibodies against an extracellular domain of this protein disrupt T cell/B cell interactions.
The generation of an immune response against an antigen is carried out by a number of distinct immune cell types which work in concert in the context of the particular antigen. An antigen introduced into the immune system first encounters an antigen presenting cell. An antigen presenting cell processes the antigen and presents antigenic fragments to helper T cells (TH), which, in turn, stimulate two types of immune responses; i.e., cellular and humoral immune responses. TH respond to antigen stimulation by producing lymphokines which xe2x80x9chelpxe2x80x9d or activate other effector cell types in the immune system. TH activate B cells to secrete antibodies which function as the major effector molecule in humoral immune responses. Antibodies neutralize foreign antigens and cooperate with other effector cells in mediating antibody-dependent cellular cytotoxicity. Additionally, TH regulate cellular immune responses by stimulating another T cell subset to develop into antigen-specific cytotoxic effector cells, which directly kill antigen-expressing target cells.
TH are distinguished from cytotoxic T lymphocytes (CTL) and B cells by their expression of a cell surface glycoprotein marker termed CD4. In the mouse, type 1 helper T cells (TH1) produce interleukin-2 (IL-2) and xcex3-interferon (xcex3-IFN) upon activation by an antigen presenting cell, while type 2 helper T cells (TH2) produce IL4 and IL-5. Based on the profile of lymphokine production, TH1appear to be involved in promoting the activation and proliferation of other T cell subsets such as CTL, whereas TH2specifically regulate B cell proliferation and differentiation, antibody synthesis, and antibody class switching.
CTL express the CD8 surface marker. Unlike most TH, these cells display cytolytic activity by direct contact with target cells, although they are also capable of producing certain lymphokines. In vivo, these cells are particularly important in situations where an antibody response alone is inadequate. There is a preponderance of experimental evidence that cellular immune responses play a principal role in the defense against viral infections and cancer.
In the immune system, immune cells communicate with each other by direct contact via surface proteins and by secretory cytokines which bind surface receptors. In most cases, cell surface molecules are distributed evenly throughout the cell membrane. However, certain cell surface proteins have been shown to cluster after lymphocyte activation. For example, an antigenic fragment presented by an antigen presenting cell brings together the T cell receptor (TCR) and other co-receptors into a complex.
Cellular polarity reflects specialization of the cell surface membrane into domains that allow cells to assess and respond quickly to their environment (Drubin and Nelson, 1996, Cell 84: 335-44). In the immune system, migrating T lymphocytes exhibit functional polarity (Negulescu et al., 1996, Immunity 4: 421-30). Cells that encounter antigen at their leading edge readily activate, whereas those that encounter antigen at the uropod do so much more poorly.
Since TCR density does not seem to be greater at the cell""s leading edge prior to antigen activation, other molecule(s) may be responsible for this intrinsic polarity. Several cytoplasmic molecules display polar distribution in lymphocytes before antigen activation. For instance, spectrin, ankyrin, and the microtubule-organizing center (xe2x80x9cMTOCxe2x80x9d) demarcate a structural pole in T cells that has been suggested to be important in the directional delivery of signaling molecules after cell-cell coupling (Geiger et al., 1982, J. Cell Biol. 95: 13743; Gregorio et al., 1994, J. Cell Biol. 125: 345-58; Kupfer et al., 1986, J. Exp. Med. 163: 489-98; Kupfer et a., 1994, J. Exp. Med. 179:1507-15; Lee et al., 1988, Cell 55: 807-16). However, prior to the present invention, a cell surface molecule had not been identified to have a polar distribution on lymphocytes before antigen activation.
A novel mammalian cell surface molecule is provided, designated cadherin-like asymmetry protein-1 (Clasp-1). In particular, polynucleotides comprising coding sequences for Clasp-1, polynucleotides that selectively hybridize to Clasp-1 coding sequences, expression vectors containing such polynucleotides, genetically-engineered host cells containing such polynucleotides, Clasp-1 polypeptides, Clasp-1 fusion proteins, therapeutic compositions, Clasp-1 domain mutants, antibodies specific for Clasp-1, methods for detecting the expression of Clasp-1, and methods of inhibiting an immune response by interfering with Clasp-1 function. A wide variety of uses are encompassed by the invention, including but not limited to, treatment of autoimmune diseases and hypersensitivities, prevention of transplantation rejection responses, and augmentation of immune responsiveness in immunodeficiency states.
The invention is based, in part, on Applicants"" discovery of Clasp-1 as a type I transmembrane protein containing certain cadherin domains and other protein domains known to be involved in signal transduction. Clasp-1 is expressed in lymphoid tissues and the brain, but is undetectable in most major adult organs. In particular, Clasp-1 is expressed in both T and B cells, as well as macrophages. The cell surface distribution pattern of Clasp-1 in lymphocytes is apical, and it is localized at the pole associated with the leading edge of the cell. More importantly, Clasp-1 is concentrated at the interface between T cell/B cell clusters, and antibodies directed to its extracellular domain inhibit T cell/B cell interactions.